First this post is a very valuable source of information for bolt replacement : Common-honda-bolt-thread-pitch-size-dimension-1885652/
I have many rusty bolts in my engine bay, i was looking at alternatives options to OEM.


This design came often, with the hex in the center. Is it solely for aesthetic purposes, or is it easier to remove ?
Also as i read most of our engine bay bolts are M6x10mm
Is it possible to put oversized M6x16mm or M6x25mm as long as the diameter is M6 ?

 

Also from a design standpoint why does OEM bolts have double washers ?